Hong-Jie Tao

Observation of pseudogap in Bi2Sr2CaCu2O8+δ single crystals with electron tunneling spectroscopy

Abstract We report electron tunneling evidence for a pseudogap in the normal state excitation spectrum of as-grown Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) single crystals. The normalized conductance curves show that the gap structure does not disappear at T c but at much higher temperature T ∗ ; the lower bound of T ∗ is within 110 K to 150 K for the investigated three junctions. The result gives a support for the existence of paired carriers in the normal state of cuprate superconductor Bi2212.

Surface pattern and large low-field magnetoresistance in La0.5Ca0.5MnO3 films

Formation of an ordered surface structure in La0.5Ca0.5MnO3 films due to the mismatch of the thermal expansion coefficient between the film and the substrate has been investigated. The surface pattern consists of grain chains located on regular orthogonal cracks. The cracks serve as weak-link grain boundaries, and unusually enhanced low-field magnetoresistance (−14.4% in 400 Oe at 90 K) has been observed, which may be explained by spin-polarized tunneling across the grain boundaries.

Josephson tunneling in MgB2 break junctions

Josephson effects in break junctions of MgB2 sintered at ambient pressure, have been investigated. The measured I-V characteristics of the junctions and the current step heights of the Shapiro steps can well be fitted with the resistively shunted junction model. Relatively large IcRn was obtained, which ranges from 1.4 to 4.2 mV. The temperature dependence of the Josephson critical current indicated that our break junctions were of the SNS weak link. The apparent deviation in the Fraunhofer pattern implied inhomogeneous current flow in the MgB2 break junctions

A new semi-Heusler ferromagnet NiFeSb: electronic structure, magnetism and transport properties

We report on structural, magnetic, transport, and spin polarization measurements of a new semi-Heusler alloy NiFeSb, which was synthesized successfully by means of electron melt-spinning technique although its ferromagnetic state and crystal structure was predicted to be unstable by the band calculation. The temperature-dependent measurement of magnetizations can both be interpreted well by the spin-wave theory. The electrical resistivity follows a T-5 behavior at low temperatures. Point contact Andreev reflection measurements of the spin polarization yield moderate polarization of 52%

Josephson effects in MgB2/Nb point-contacts

We have systematically investigated the dc and ac Josephson effects in MgB2/Nb point-contacts. The observed I–V characteristics of the point-contacts can be described nicely by the resistively shunted junction (RSJ) models and large characteristic voltages, 2–3 mV, have been obtained. The dependence of the Josephson current step heights on the microwave current amplitude can also be fitted by both dc and ac current-biased RSJ models. The measured temperature dependence of the Josephson critical current agrees well with the Ambegaokar and Baratoff equation at low temperature regions, but there are deviations from it at high temperatures. These results imply that the 39 K MgB2 is more likely a BCS superconductor and has its potential application in superconducting electronics.

Epitaxial growth of Bi2Sr2-xLaxCuO6+δ thin films by RF-magnetron sputtering

Superconducting Bi2Sr2-xLaxCuO6+delta thin films were successfully deposited on (100) LaAlO3 and (100) SrTiO3 substrates by in situ RF-magnetron sputtering. The highest zero resistance temperature T-c0, reached 29 K. The film morphology is shown to be related with the sputtering gas pressure. The microstructures. of the films show ab-twinning. Atomic force microscopy image showed the films grow as terrace islands with no screw dislocation. For obtaining high quality La-2201 superconducting thin films, the sputtering gas pressure should be higher, and the compositions of the targets have to be modified by trial and error

Epitaxial growth of Bi2Sr2−xLaxCu1O6+δ thin films on vicinal SrTiO3 substrates

Abstract We have studied the epitaxy of Bi 2 Sr 2− x La x CuO 6+ δ (La-2201) superconducting thin films deposited by RF sputtering on vicinal SrTiO 3 (STO) substrates. The surface of the single crystal substrate is cut at an angle of 6° with respect to the (100) basal plane, being rotated around the [110] axis direction. Contrary to films deposited on untilted single crystal substrate, the AFM surface topographies of the La-2201 films deposited on the tilted substrates show elongated stripe-like shapes. Transmission electron microscopy (TEM) and electron diffraction show a film growth with very good crystallographic alignments very similar to what is observed on La-2201 single crystals. The temperature dependency of the a - and c -axis resistivities confirm the high epitaxial quality of the films, as evidenced by atomic force microscopy (AFM) and TEM studies.

Atomic Force Microscopy of Surface Reconstructed SrTiO3 Vicinal Substrates for Epitaxial Growth of YBa2Cu3O7-δ Thin Films

SrTiO3(001) vicinal substrates of miscut 1 degrees, 3 degrees, and 6 degrees toward the (001) plane were annealed at 800-1000 degrees C in pure flowing O-2 for 6 h and examined by atomic force microscopy. The high temperature annealed vicinal SiTiO3 (001) displayed arrays of straight steps and smooth terraces. The step heights vary from about 2 to 6 unit cell depending on the annealing temperatures. At 1000 degrees C, step bunching becomes obvious. The height steps of 1 unit cell are difficult to be observed by our atomic force microscopy. The YBa2Cu3O7-delta thin film grows epitaxially on the step-terrace surface with c-axis orientation by magnetron sputtering in the two-dimensional island mode. Prospect of step-flow growth is being discussed.

AFM study of growth of thin films

c-axis-oriented thin films deposited on flat planes of , and (100)MgO substrates and vicinal planes (off-angle ) of substrates by RF magnetron sputtering were studied by atomic force microscopy (AFM). of these films reached 29 K. Film thickness ranged from 15 nm to 600 nm. Two typical growth modes have been observed. AFM images of thin films on flat planes of substrates showed a terraced-island growth mode. By contrast, Bi-2201 thin films on vicinal planes of substrates showed a step-flow growth mode. The growth unit is a half-unit-cell in the c-axis for both growth modes. No example of spiral growth, which was thought to be the typical structure of YBCO thin films, was found in either of these kinds of thin films.

Tunneling observation of the coexistence of two distinct gaps in Bi2Sr2CaCu2O8+δ

Abstract Temperature, T , and doping dependent tunneling spectroscopy on Bi 2 Sr 2 CaCu 2 O 8+ δ /natural barrier/Zn thin film (Bi2212/I/Zn) junctions has been investigated systematically. For most of the junctions, the tunneling conductance shows one superconducting gap (SG) at low T and a discontinuous transformation from the SG to the pseudogap (PG) at T c . For some junctions studied, the conductance reveal two distinct gaps: an inner gap which closes when T approaches to T c with BCS-like T -dependence, and an outer gap, coexisting with the inner gap for T below T c , which is almost T -independent below about 120 K. We discuss the possible origins of the coexistence of the two gaps, including the coexistence of the SG and PG and the gaps resulting from electronic or doping inhomogeneity.